My personal recommendation is to develop a larger body motion to keep time. So, if you think about the time it takes to strums from the bottom of the strings to the top, how much time do you then have to pause between strums? What can you do to fill that gap to shrink the pause and enable you to have a consistent motion? This is the sort of methodology I use to help me keep good time as a drummer. I guess in musical terms you could think of it as a legato playing style. Obviously drums are a percussive instrument and have a natural decay that isn't manipulated in the same way as notes on a piano, but instead the legato applies to the stroke, not the note itself.

Does that make sense? I know what I'm talking about in my head at least :)